This invention pertains to the art of plate-type heat exchangers which comprise a stack of parallel plates spaced to provide passageways therebetween. Heat is transferred from the fluids passing in one set of passageways to the fluids passing in another set of passageways via heat conduction through the interposed plates. More particularly this invention relates to plate-type heat exchangers in which the passageways are provided with sections of corrugated fin material or packing. Such heat exchangers have been employed for rectification purposes. Gas or vapor is caused to flow upward through one passageway in countercurrent flow relationship to a liquid. Heat is transferred to or from this liquid and vapor by another passageway for nonadiabatic rectification. The corrugated fin packing improves the heat transfer and permits extensive vapor-liquid contact for mass transfer. Ideally, the composition of the fluids in the one passageway will progressively change from one end of the heat exchanger passageway to the other as a result of the constituent concentration differentials between the contacting liquid and vapor and heat transfer acting over a period of time. An example of a plate-type heat exchanger intended for use in this manner may be found in U.S. Pat. No. 2,703,700.
Plate-type heat exchangers with corrugated fin packing used for nonadiabatic rectification have followed at least two designs. As used herein, "corrugated fin packing" means a packing formed by corrugating a porous or nonporous metallic sheet.
In the first type of heat exchanger the principal corrugated fin packing for the two-phase flow passageway is arranged so that the crests and valleys thereof extend substantially vertically except for the distributor sections adjacent the inlet and outlet. An example of such a design may be found in U.S. Pat. No. 3,568,461. In the second type of heat exchanger the principal corrugated fin packing for the two-phase flow passageway is arranged so that the crests and valleys extend substantially horizontally except for the distributor sections adjacent the inlet and outlet. An example of such a design may be found in U.S. Pat. No. 3,568,462.
Thus, in the first type of heat exchanger the general fluid flow is parallel to the crests and valleys of the corrugated fin packing. The liqud and vapor passing in intimate contact with each other pass the "easy way" through the corrugated fin packing. In the second type of heat exchanger the general fluid flow is normal to the crests and valleys of the corrugated fin packing. Thus the liquid and vapor passing in intimate contact with each other pass the "hard way" through the corrugated fin packing. In the "hard way" design the packing must be porous so that the fluid may pass through the sheet material of the corrugated fins.
Each of these two basic design approaches, I believe, present certain difficulties for which the instant invention is intended to correct. I believe the first mentioned type of heat exchanger is predisposed to pass the liquid too quickly toward the bottom of the passageway thereby resulting in insufficient dwell time for the contacting vapor and liquid. I believe an improved vertical distribution or slower fall of the liquid is required to obtain an efficient mass transfer throughout the heat exchanger passageway to permit a gradual change in composition of the fluids from one end of the passageway to the other. Attempts to obtain better vertical distribution by increasing the velocity of upward vapor flow as by decreasing the fin spacing may result in objectionable liquid carry-over. Liquid carry-over then establishes a limit to the throughput.
With respect to the second mentioned type of heat exchanger, I have discovered that for the pores within the porous fin packing to be sufficiently small to adquately uphold the liquid portion for increasing the dwell time by obtaining a wide vertical distribution of liquid, the passageway is severely limited in liquid throughput. Some designs have attempted to solve this problem by using substantially larger pores located to form liquid holding pockets throughout the passageway. An example of this construction will be seen in U.S. Pat. No. 3,512,262.
Furthermore, attempts have been made to combine the two aforementioned designs in an effort to maintain more uniform horizontal distribution of gas and liquid across the width of the two-phase passageway by the use of a liquid redistributor. Such a design is shown in U.S. Pat. No. 3,612,494.
In all the aforementioned constructions, the liquid may tend to move both upward by force of the vapor flow and downward by force of gravity. There is a need for a net downward movement of the liquid because the liquid is introduced to the passageway near the top and removed near the bottom. In these prior art designs, this net downward movement of liquid is in counterflow relationship with the vapor passing upwardly. For the liquid and vapor to pass each other in this manner, a larger passageway is required than had the liquid and vapor been in separate channels. I believe this net downward flow of liquid within the interstices of the fin packing in counterflow relationship to the upward vapor flow is thus sufficient to substantially reduce the potential throughput capacity. This is because the liquid within the interstices of the fin packing tends to become entrained with and carried along with the vapor and because a larger passageway is required.
It will thus be seen that prior art designs may have encountered numerous problems including: insufficient liquid dwell time, non-uniform horizontal distribution of liquid and vapor, inadequate liquid vapor contact, liquid carryover, insufficient vertical spread of liquid, and restrictive throughput capacity.